An aliquot of urine is acidified and mesoporphyrin is added as an internal standard. Porphyrins in the acidified urine are separated by HPLC and the eluted porphyrins and internal standard are quantified by comparison of their fluorescence intensity to that of known porphyrin standards.(Ford RE, Ou C-N, Ellefson RD: Liquid chromatographic analysis for urinary porphyrins. Clin Chem 1981;27:397)